There are many sensor application areas where there is a clear need for innovative power solutions. The market for wireless sensor networks in industrial automation, supply chain management, construction, home automation, asset tracking and environmental monitoring is expected to grow to well over 400 million devices by 2012. The average useful life of such a system is targeted to be more than 10 years, which means that the stand-alone usage of conventional batteries poses significant barriers to being a robust energy solution.
Harvesting energy from motion has been the focus of intense research. There are three common technological approaches: piezoelectric, electrostatic, and electromagnetic. Numerous research groups and companies have tried to develop miniature thin-film piezoelectric devices to harness vibrations in the last 20 years. However, one problem is that thin-film piezoelectric energy have limited power output because of their high-voltage low-current output, typically tens of volts and less than nanoamperes, which makes it difficult to convert without substantial losses. Another problem is the high intrinsic frequencies of piezoelectric (PZT) materials, typically around MHz, that can't be coupled to any vibrations or cyclical motion available for practical applications.
Other groups have focused on developing electrostatic generators. Electrostatic generators have limited power output similar to piezoelectric generators also due to the fact that they produce only high voltages and low electrical currents. Furthermore, it can be shown that in most cases electrostatic generators have lower power densities than either piezoelectric or electromagnetic generators due to the relatively low energy density of an electrostatic air gap on which the electrostatic generators rely.
On the other hand, electromagnetic power generators have the potential to supply relatively large amounts of power without being restricted to the intrinsic frequencies of piezoelectric materials. However, generating sufficient power at a desired compact scale has still not been achieved. Further, the unmatched natural frequency of a small scale device, typically kHz, cannot be coupled to the vibrations that are commonly available for most applications. Lastly, current designs require state-of-the-art precision machining and assembly (e.g., e laser cutting, electrical discharge machining (EDM), and CNC machining) or micromachining and thin film technologies (e.g., Magnetic materials, both permanent magnets and magnetic alloys, are difficult and expensive to do as thinfilms. Micromachining in general gets expensive as the size of the device gets larger, and in this case the devices need to be relatively large (˜1 cm^2) to give any reasonable amount of power. At that size, micromachining becomes quite expensive.) that drastically raise manufacturing costs beyond that of batteries.